The present invention relates to a regulating resistor network for regulating voltage or current supplied to a semiconductor circuit within certain limits, a semiconductor device including such a resistor network and a method for fabricating the device.
Hereinafter, a semiconductor device including a conventional regulating (or trimming) resistor will be described with reference to FIGS. 10(a) and 10(b).
As shown in FIG. 10(a), first and second chips B and C are mounted on a printed wiring board A. A field effect transistor (FET) 1 is formed as an amplifier on the first chip B. The drain terminal of the FET 1 is connected to a drain voltage supply terminal 2, while the source terminal thereof is grounded.
A resistance adjustment circuit 4 for regulating a bias voltage applied to the gate terminal 3 of the FET 1 is formed on the second chip C. The resistance adjustment circuit 4 includes a voltage supply terminal VGG, first and second voltage dividers 5 and 6 and a trimming resistor 7, which are connected in series to each other. In FIG. 10(a), a signal input terminal VS for applying an input voltage to the gate terminal 3 of the FET 1 is provided on the printed wiring board A.
As shown in FIG. 10(b), the trimming resistor 7 is implemented as a thin-film resistor including a plurality of notches 7a, which have been formed by laser-trimming the thin-film resistor. In such a thin-film resistor, the current path has an increased length. Thus, the resistance value of the thin-film resistor goes on increasing gradually as the current flows over a longer distance, and the resistance value of the trimming resistor 7 is adjustable.
Once the resistance value of the trimming resistor 7 has changed, the amount of a current flowing through the resistance adjustment circuit 4 also changes. As a result, the bias voltage applied to the gate terminal 3 of the FET 1 changes, too. Thus, even if the operating current of the FET 1 has deviated from a predetermined value, the operating current can be regulated at the desired value by adjusting the resistance value of the trimming resistor 7 and thereby changing the bias voltage applied to the gate terminal 3.
Hereinafter, a method of regulating the operating current of the FET 1 by adjusting the resistance value of the trimming resistor 7 will be briefly described.
First, a predetermined voltage is applied to the voltage supply terminal VGG of the resistance adjustment circuit 4 and a bias voltage is applied to the gate terminal 3 of the FET 1. In such a state, while the operating current of the FET 1 is being monitored, the thin-film trimming resistor 7 is laser-trimmed, thereby gradually increasing the resistance value of the trimming resistor 7.
Thereafter, when the operating current flowing through the FET 1 reaches a predetermined value, the laser trimming of the thin-film resistor is stopped, thereby fixing the current flowing through the resistance adjustment circuit 4 at a constant value.
As described above, to adjust the resistance values of the trimming resistor 7 and the resistance adjustment circuit 4, the thin-film trimming resistor 7 should be laser-trimmed, while the operating current of the FET 1 is being monitored. Thus, it usually takes as long a time as 5 to 7 seconds to complete such fine adjustment.
In addition, since the resistance value of the thin-film trimming resistor 7 should be adjusted by irradiating the trimming resistor 7 with laser beams for such a long time, the trimming resistor 7 and the resistance adjustment circuit 4 both receive a large quantity of heat. Accordingly, if the resistance adjustment circuit 4 and the FET 1 are formed on the same chip, then the laser irradiation on the trimming resistor 7 causes considerable damage to the FET 1.
Thus, in the conventional arrangement, a semiconductor circuit such as the FET 1 and the resistance adjustment circuit 4 should be separately formed on the first and second chips B and C. And then these chips B and C should be mounted on the printed wiring board A. That is to say, the semiconductor circuit and resistance adjustment circuit cannot be integrated on the same chip.
An object of the present invention is getting resistance adjustment circuit and semiconductor circuit integrated on the same chip by shortening the time taken to adjust the resistance value of the resistance adjustment circuit and by reducing the quantity of heat received by the resistance adjustment circuit during the adjustment.
A regulating resistor network according to the present invention includes a plurality of resistors connected in parallel to each other. Each said resistor is cuttable by being irradiated with light, and a resistance value of the regulating resistor network is adjustable by cutting at least one of the resistors off.
In the inventive regulating resistor network, each resistor is cuttable by being irradiated with light, and therefore, if at least one of the resistors is cut off, then the resistance value of the regulating resistor network is adjustable. Thus, the time taken to adjust the resistance value of the regulating resistor network can be shorter than the conventional one. That is to say, there is no longer any need to change the length of a current path in a thin-film resistor through notching as is done in the conventional trimming resistor. Instead, the resistance value is changeable only by cutting at least one of the resistors off according to the present invention. Thus, the time needed for adjusting the resistance value can be drastically cut down.
In one embodiment of the present invention, the resistors preferably have mutually different resistance values. In such an embodiment, a great number of resistance value combinations can be created by cutting off a variable number of resistors.
In this particular embodiment, the resistors are preferably of mutually different lengths. In such an embodiment, even if a size error occurred among the resistors formed by patterning a metal thin film, the resistance value of the regulating resistor network does not deviate from its desired value so much and is still finely adjustable.
In another embodiment of the present invention, the resistor network preferably further includes a fixed resistor, which is connected in parallel to the resistors and is not cuttable by being irradiated with the light.
In such an embodiment, even when the resistance values of the resistors are distributed within a wide range, the difference between resistance values obtained by cutting at least one of the resistors off can be reduced.
A semiconductor device according to the present invention includes: a semiconductor substrate on which a semiconductor circuit is formed; and a regulating resistor network, formed on the semiconductor substrate, for regulating a voltage or current supplied to the semiconductor circuit. The regulating resistor network includes a plurality of resistors connected in parallel to each other. Each said resistor is cuttable by being irradiated with light, and a resistance value of the regulating resistor network is adjustable by cutting at least one of the resistors off.
In the semiconductor device according to the present invention, the resistance value of the regulating resistor network is adjustable only by cutting at least one of the resistors off, and therefore, the time taken to adjust the resistance value of the resistor network can be much shorter. In addition, the light irradiation time and heat quantity required for the fine adjustment of the resistance value can also be reduced. Thus, even if a semiconductor circuit is integrated with the regulating resistor network on the same semiconductor substrate, the semiconductor circuit receives much smaller damage. Accordingly, it is possible to integrate the semiconductor circuit with the regulating resistor network for regulating the voltage or current supplied to the semiconductor circuit on the same semiconductor substrate.
In one embodiment of the present invention, the resistors preferably have mutually different resistance values. In such an embodiment, a great number of resistance value combinations can be created by cutting a variable number of resistors off.
In this particular embodiment, the resistors are preferably of mutually different lengths. In such an embodiment, even if a size error occurred among the resistors formed by patterning a metal thin film, the resistance value of the regulating resistor network does not deviate from its desired value so much and is still finely adjustable.
In another embodiment of the present invention, the resistor network preferably further includes a fixed resistor, which is connected in parallel to the resistors and is not cuttable by being irradiated with the light.
In such an embodiment, even when the resistance values of the resistors are distributed within a wide range, the difference between resistance values obtained by cutting at least one of the resistors off can be reduced.
In still another embodiment, the semiconductor circuit may be a field effect transistor, and the regulating resistor network is preferably connected to the gate terminal of the field effect transistor. And a bias voltage applied to the gate terminal of the field effect transistor is preferably regulable by adjusting the resistance value of the regulating resistor network.
In such an embodiment, the field effect transistor and the regulating resistor network for regulating the gate bias voltage of the field effect transistor can be integrated on the same semiconductor substrate.
An inventive method for fabricating a semiconductor device includes the step of a) forming a semiconductor circuit and a regulating resistor network on a semiconductor substrate. The resistor network includes a plurality of resistors and regulates a voltage or current supplied to the semiconductor circuit. Each said resistor is cuttable by being irradiated with light. The method further includes the step of b) cutting at least one of the resistors off by irradiating the resistor with the light, thereby adjusting a resistance value of the regulating resistor network.
According to the method of the present invention, it is possible to shorten the time needed for adjusting the resistance value of the regulating resistor network. In addition, a semiconductor device, in which a semiconductor circuit is integrated with the regulating resistor network on the same semiconductor substrate and yet receives little damage during the adjustment of the resistance value, can be fabricated just as intended.
In one embodiment of the present invention, the resistor network preferably further includes a fixed resistor, which is connected in parallel to the resistors and is not cuttable by being irradiated with the light.
In such an embodiment, even when the resistance values of the resistors are distributed within a wide range, the difference between resistance values obtained by cutting at least one of the resistors off can be reduced.
In another embodiment, the light for cutting at least one of the resistors off is preferably pulsed YAG laser radiation.
In such an embodiment, the YAG laser radiation is easily absorbable into the resistors in the network, but hardly absorbable into the semiconductor substrate. Thus, the semiconductor circuit, which is formed on the same semiconductor substrate as the regulating resistor network, receives lesser damage upon the irradiation of the laser radiation. Also, since the YAG laser radiation is pulsed, damage done to the semiconductor circuit by the laser irradiation is even smaller.
In still another embodiment, the step b) preferably includes irradiating the regulating resistor network with the light through a mask. The mask preferably includes a plurality of openings at respective locations corresponding to the resistors. Some of the openings associated with the at least one resistor to be cut off should be opened but the other openings associated with the remaining resistors not to be cut may be closed during the irradiation of the light.
In such an embodiment, no matter how many resistors should be cut off, those resistors can be cut simultaneously and accurately.
In still another embodiment, the step b) preferably includes measuring an operation characteristic value of the semiconductor circuit and cutting at least one of the resistors off so as to reduce a difference between the characteristic value measured and a target characteristic value preset for the semiconductor circuit.
In such an embodiment, the process step of adjusting the resistance value of the regulating resistor network can be incorporated into a test process for the semiconductor circuit. Thus, the process step of adjusting the resistance value of the regulating resistor network can be performed on the semiconductor substrate on a semiconductor wafer.
In still another embodiment, the semiconductor circuit may be a field effect transistor, the regulating resistor network is preferably connected to the gate terminal of the field effect transistor, and the step b) preferably includes regulating a bias voltage applied to the gate terminal of the field effect transistor.
In such an embodiment, the field effect transistor and the regulating resistor network for regulating the gate bias voltage of the field effect transistor can be integrated on the same semiconductor substrate.